Fix: workqueue: futex wait: handle spurious futex wakeups

[urcu.git] / src / workqueue.c

diff --git a/src/workqueue.c b/src/workqueue.c
index e5931b5e74bfee68c3efd1ff61b53e9bc68af23f..c9f03b56fbb3ac85c58e5ef21330e03d0377d340 100644 (file)
--- a/src/workqueue.c
+++ b/src/workqueue.c
@@ -129,18 +129,29 @@ static int set_thread_cpu_affinity(struct urcu_workqueue *workqueue __attribute_
 
 static void futex_wait(int32_t *futex)
 {
+       int ret;
+
        /* Read condition before read futex */
        cmm_smp_mb();
-       if (uatomic_read(futex) != -1)
-               return;
-       while (futex_async(futex, FUTEX_WAIT, -1, NULL, NULL, 0)) {
+       while (uatomic_read(futex) == -1) {
+               if (!futex_async(futex, FUTEX_WAIT, -1, NULL, NULL, 0)) {
+                       /*
+                        * Prior queued wakeups queued by unrelated code
+                        * using the same address can cause futex wait to
+                        * return 0 even through the futex value is still
+                        * -1 (spurious wakeups). Check the value again
+                        * in user-space to validate whether it really
+                        * differs from -1.
+                        */
+                       continue;
+               }
                switch (errno) {
-               case EWOULDBLOCK:
+               case EAGAIN:
                        /* Value already changed. */
                        return;
                case EINTR:
                        /* Retry if interrupted by signal. */
-                       break;  /* Get out of switch. */
+                       break;  /* Get out of switch. Check again. */
                default:
                        /* Unexpected error. */
                        urcu_die(errno);